Magnetic pickup



Aug. 27, 1940.

j INVENTOR 3i? l g l /llanzzfm'an Weil BY f ATTORNEYS M. WEIL v2,212,530

MAGNETI C PI CKUP Filed Sept. 4, 1936 scribed as hang-over.

.Patented Aug. 27, 1940 UNITED sTArss 21 Claims.

My present invention relates to magnetic` pickups for phonographicreproduction.

As conducive to a clear understanding of the invention, it is noted thatthe moving armature mass of magnetic pickups generallyr has a naturalfrequency of vibration within that audible frequency range required forsatisfactory performance, with consequent marked damping out offrequencies above that of resonance. Rapid changes in amplitude offrequency in the normal use of such device due to its considerableinertia result in a distorted performance, commonly de- While damping ofthe armature movement alleviates these difliculties to some extent,great increase in stylus-point impedance is thereby entailed, withconsequent reduction in the eliiciency of the unit and marked increasein the wear and tear Von the sensitive record groove.

Among the objects of the invention are to provide a magnetic pickupunit, the performance of which approximates that of the highly de-`veloped microphone used in recording, so that the output of said pickupis practically indistinguishable from the original performance asrecorded.

Another object is to provide an instrument of the above type, which issimple, sturdy and compact in construction7 and in which the inertia ofthe moving system is so low that its natural frequenc is outside,preferably beyond that of the useful range of audibility, withconsequent uniform delicacy of response substantially throughout theeffective audible range.

Another object is to provide an instrument of the above type, in whichthe light moving mass is mounted effectively to control the flux throughthe magnetic elements without weavingor wobbling and in which theresistance imposed to vibration is so slight that the weight resting onthe record, and the wear on the record groove are greatly reduced.

Another object is to provide an instrument of the above type whichrequires no adjustment in j use for adaptation to various types ofrenditions ,and which maintains its initial sensitiveness and efficiencyfor long periods of use.

According to the invention, the vibratory system of the instrument, thatis, the vibrating magnetic element and associated moving parts by `whichthe uX is controlled in synchronism with the vibrations imposed by therecord upon the stylus is greatly reduced'in inertia. To this end, themagnetic circuit is so contrived that a fixed magnetic element havingonly a small fraction of the mass of the usual armature actuallyoscillate's, and serves as a magnetic flux-relay piece, the relativelygreater bulk and mass of the armature proper being anchored againstvibration.

In a desirable construction, the magnetic vflux relay piece ispositioned in the gap between the field poles and the armature and iscarried preferably by a non-magnetic extremely light shaft supported `inthe casing and carrying the stylus. The predominating part of the massof the vibratory assembly is the stylus itself, the rest of the partsbeing extremely light in mass and moving in low friction bearings, whilethe stylus itself is preferably of special construction, affording avminimum of mass at its point and end with the' major portion of itssmall mass at the central clamped portion thereof. f i

In the accompanying drawing in which are shown one or more of variouspossible embodiments of the several features of the invention,

Fig. l is a perspective view of the completely assembled unit,

Fig. 2 is a view in-longitudinal cross section thereofktaken on line 2 2of Fig. 1 and on a larger scale, 4

Fig. 3 is a plan View of the device with the cover removed, parts beingshown in section,

Fig. 4 is a transverse sectional view taken on line d-Li of Fig. 2, vFig. 5 is a transverse cross sectional view taken along the line 5 5 ofFig. 2,

Fig. 6 is a perspective View of the pole pieces and armature incorrelated position,

Fig. '7 is a perspective view of the magnetic l ux relay assembly,

Fig. 8 is a perspective view of the stylus shown in the embodiment ofFigs. 1 to 7,

Fig. 9 is a fragmentary View in longitudinal section of the forward endof the casing showing an alternative sylus,

Fig. l is a perspective view of the stylus shown in the embodiment ofFig. 9,

Fig. 11 is a fragmentary detail view of a modification, and

Fig. l2 is a similar view 'of another modification. i

Referring now to the drawing, the pickup comprises a casing including abase iii specially molded to accommodate the parts mounted therein andhereinafter described, and equipped with a sheet metalicover iltelescoped over the sides thereof and preferably tapering downward atits front as at liand attached by screws i3. The shank M of the casinghas terminal sleeves l therein for the terminal members I6 protrudingtherefrom, from which insulated conductors il lead to theelectro-magnetic coil i8 known as the Voice coil.

Upon the base is mounted the horseshoe magnet l closely fitting againstthe sides of the base and contacting at the forward end of its legs, themagnetic pole pieces 2li and 2i which are attached to the base by meansof screws 22 countersunk in the underiace of the base BG. The magnet is:retained in place by means of a leaf spring 23 preferably of copperalloy reacting at its ends against shoulders il molded in the base, witha central bowed part reacting against the extremity of the horseshoemagnet, and pressing it forward against the pole pieces. Preferably thespring is longitudinally slit at on a level with the upper face of themagnet-and the strap 26 determined thereby is pressed forward to overlapthe magnet and retain it against the base lil.

The pole pieces are preferably U-shaped as shown'with their upper legsor ledges 2l aligned, but spaced from each other as shown, and theirlower legs or ledges t similarly aligned and spaced. The spacedetermined between ledges of the facing pole pieces accommodates the vcoil I8.

A xed armature piece 2d extends downward centrally-of the coil, asshown, is symmetrically spaced from the upper ledges 2l of the polepieces and its lower end is slightly above the lower ledges 28 of thepole pieces. Said ixed armature is mounted by screws 3Q to ar plate 3lof non-magnetic material, which, in turn, is attached by means of screw32 to the upper ledge of the pole piece Z0. While the specicconstruction and arrangement shown is preferred, it is manifest that thecoil could be disposed to encircle any part of the magnetic circuitother than the armature, if desired.

The magnetic flux relay comprises an extremely light piece of magneticmaterial, including a shank 33 with an upstanding iiattened wing 34E thelatter disposed centrally between the lower ledges 28 of the polepieces, preferably to protrude slightly above ledges 28 and slightlybelow the lower extremity of the fixed armature 29 so as to determinesmall predetermined air gaps with respect to said three pieces, as bestshown in Fig. 4. This relay piece is mounted by frictionally fitting itsshank 33 into a shaft or tube 35 Vof extremely light material,preferably of the alloy known as Duralurninf which extendslongitudinally of the base piece and is mounted in bearings 36 and 3l atopposite ends thereof. Preferably, rubber bushings- 3d encircle the endsof the tube, which are mounted upon the base, by bearing cap pieces 39attached to the base by screws lili.

In order to maintain the wing piece normally in, properly centeredposition, the sleeve 3&5 is provided with an upstanding pin or stud iliixed therein, with a flattened upper extremity 132 extending into acorresponding notch it on a stop piece 44 i'l'xed by screws i5 upon acorresponding ledge d6 molded in the base lil.

The stylus il extends transversely through corresponding apertures lilin the base l@ and aper- 'tures in sleeve 35 and bearing ill and isclamped at its median portion against the wall of the tube apertures bymeans of a set screw 5@ extending through the forward end of the baselll and threaded into the forward end of the tube 35.

While the stylus may be of conventional `construction, it is preferredto employ the structure said V shown in Fig. 8 in which the main shankof the stylus is of conventional diameter, but the pointed end 5ithereof is not the ordinary conical point but is concave longitudinallyas shown, for reduction of mass and for effecting a particularly sharppoint. Moreover, the upper part 52 of the stylus is of greatly reduceddiameter for further reduction in mass. By this construction, thoseparts of the stylus, namely, the parts near the lower and upper ends,which perform the major movement in operation, are of greatly reducedmass, while the major part of the mass is that intermediate portion inthe region clamped by the set screw lli? which performs relativelyslight movement, whereby the inertia of the stylus is effectivelyreduced. Preferably the stylus is insertedthrough an aperture 53 in thecover l l and its position is thus accurately determined as shown inFig. 2, by bringing the upper end of the stylus flush with the outerwall of the cover ll.

The entire assembly of Duralumin tube and magnetic flux relay as shownin Fig. 7 is of almost negligible weight, as little as 11A-2 grains, thepreponderating inertia ci thevibratory parts in the device residing inthe stylus 565 in a preferred construction.

In use ci the device on the conventional laterally cut disk record, itwill be seen that the vibrations of the stylus point lll effectcorresponding rocking of the tube 35 in its bearings 3&5 and 3l againstthe resiliency of rubber bushings 38 which act to damp the vibration,and the wing piece of the magnetic relay in partaking of this movement'causes corresponding variations in the air gaps and controls thevariations and alternations of magnet-ic between the pole pieces 2u andil in manner readily understood by those skilled in the art. As is clearto those skilled in the art the construction may readily be embodied foruse with hill and dale records.

The sensitiveness of the device is a consequence of the fact that it isnot a relatively heavy armature which is to move in response to thevibrations of the stylus, but the control of magnetic flux is effectedby the extremely light moving element 3ft which is notan armatureproperly speaking, but may be properly designated a magnetic iiux relaypiece the movement of which determines the course of of magnitudegreater than that which traverses the relay piece itself. The naturalfrequency of the vibratory system is beyond the eiective range ofaudible frequencies. The course of the magnetic flux is substantiallyconnned as is obvious to the magnet, its pole pieces, the stationaryarmature, the relay piece and the air gaps therebetween, and in nowisereaches the stylus 5t, since the intervening tube 35 is desirably ofnon-magnetic material as set forth.

In the embodiment of Figs. 9 and 10 is shown a modification of stylus5153 which illustratively is iiattened near its upper end as at 55 toaord a stop resting against the upper portion of the forward bearing 3lof the tube 35' so that it is positively positioned with respect to thetube, as distinguished from the mode of positioning previously describedand resorted to in the embodiment of Fig. 2.

It is, of course, understood that the width of the notch i3 is such asto permit the magnetic relay wing 3Q to move laterally in the gapbetween the pole pieces without ever touching either of said pole piecesand the resilience of the rubber bushings 38 serves properly to returnthe wing piece Sil to the correctly centered Aposition shown inFig. 4,so `that no bias occurs toward one or the other pole in the absence ofimpulse applied through the stylus. If damping in addition to thatafforded by'resilient bushings 38 is required, this can conveniently beaorded by rubber stops (notl shown) interposed in the space between studextremity 42 and the walls of `notch d3 in stop piece 44. l

It is important in the initial assembly of the device that the magneticrelay wing piece 34 be correctly centered between the two pole piecesand that it be correctly spaced from the lower end of the armature piece29. For ythis purpose, resort may be had to a `two-prong fork (notshown) of width snugly to fit between the pole pieces and which has aspace between its tines `snugly to straddle the thickness of the wingpiece 34., The distance of the magnetic flux relay wing piece 34 fromthe armature 29 is determined by applying the armature after the polepieces and flux relay wing have been positioned as' ldescribed. Thearmature piece is iirst caused to engage the upper edge of the wingpiece 312i which result may be readily accomplished by looseningfastening screws'll and moving the armature up or down slightly alongits elongated screw slots `Sil' until such adjustment has occurred.

Thereupon `a shim 5t of suitable thickness, desirably in the order of@03 in thickness, is interposed between the mounting plate y3l and thepole piece 2li whereupon the screws Jill are tightened and the desireddistance between the `armature' piece and the relay piece viz'. that ofthe shim thickness is thus readily and accurately determined.

The embodiment of Fig. l1 shows a reiinement in the correlation betweenthe armature 29' and the iiux relay wing 34'.' In this case the armatureis shown with a concave lower end Gli so that in the vibratory movementof the iiux relay piece the width of the air gap with respect to thearmature will vary, decreasing in a direct ratio with the amplitude ofthe vibration. By this expedient the reluctance of the magnetic circuitautomatically decreases with the lower frequencies which correspond tothe wider amplim tude of vibration so that the low pitched notes arecorrespondinglyA accentuated, thereby to `compensate for the reductioninthe amplitude of :low pitched notes on the record frequently madenecessary by the close spacing of the spiral convolutions.

Fig. 12 shows another alternative embodiment in which the armature 292'is provided with a fork end presenting parallel lugs `lil and @7i bentween which the flux relay 342 extends. As will be seen in this casealso the air gap between the armature and the flux relay willalsodecrease in a direct'ratio of the amplitude of vibration to perform aneffect corresponding to that accomplished by the embodimentof Fig. 11.

It will thus be seen that there is herein described apparatus in whichthe several features of this invention are embodied, and which apparatus in its action attains the various objects of the invention and iswell suited to meet the requirements of practical use.

As many changes could be made in the above construction, and manyapparently widely different embodiments of this invention could be madewithout departing from the scope thereof, it is intended that all mattercontained in the above description or shown in the accompanying drawingshall be interpreted as illustrative and not in a limiting sense.

Having thus described my invention, what I claim as new andV desire tosecure by Letters Patent is:

1. A magnetic pickup having substantially xed magnetic parts withanassociated voice coil and a vibratory magnetic part, a stylus, anon-magnetic connection between the stylus andthe vibratory magnet part,said connection armature extending into said coil to the vicinity ofsaid pole pieces, a magnetic ilux relay piece intervening between saidpole pieces and near said armature, a carrying shaft therefor below saidpole pieces and perpendicular to the axis thereof, and a stylus carriedby` said shaft and mechanically connected thereby to said flux `relaypiece. i

3.` A magnetic pickup comprising a casing having axed magnet thereinwith a pair of pole pieces with upper and lower ledges, a voice coilhoused between said pole pieces, a fixed armature extending between saidpole pieces into said coil and terminating above the lower ledges ofsaid pole pieces, and a flux relay extending between said pole ledgescontiguous to said fixed armatureand a stylus and a shaft at rightangles tothe axis of said coil and mechanically connected to saidfluxrelay.

4. A magnetic phonograph reproducer com-` prising a magnet havingU-shaped pole pieces with facing legs, a voice coil symmetrically housedtherein, a fixed Varmature extending through the air gap between onepair of legs of the pole pieces and terminating at its lower andcontiguous the other air gap, a magnetic flux re-` y lay piece extendingsymmetrically in said latter air `gap and contiguous to the armatureend, a stylus and rigidvrneans at right angles to the axis of said coilVand mechanically connecting the` vstylus to said flux vrelay piece.

5. A` magnetic pickup comprising a base, a pair of pole pieces securedthereto having upper and lower ledges facing each other, a voice coildisposed between said ledges symmetrically of said pole pieces, a fixedarmature extending downward axially of said coil between the upperledges and terminating adjacent the lower ledges, a magnetic iiuxrelayrpiece disposed in the space between said lowerledges and slightlyspaced from the` lower end of said armature, a rigid shaft for said fluxrelaypiece, at right anglesto the axis of said coil, a bearing mount onsaid base for said shaft and a stylus clamped to said shaft.`

6. A magnetic pickup comprising a base, a pole piece, voice coil andfixed armature assembly thereon, a magnetic flux relay piece includingan upstanding wing, a light shaft rigidly mounting .said wing near oneend thereof, bearing means -supporting said shaft upon said base, and astylus clamped with respect to said shaft near the other end thereof.

7. A magnetic pickup comprising a base and a pole piece, voice coil andiixed armature assembly, a magnetic flux relay including an upstandingwing, a light shaft rigid with said wing and extending forwardtherefrom, a stylus ixed'with respect to said shaft, an upstanding poston said shaft and stop means laterally of said post to dampen and limitthe movement of said wing between the armature poles- 8, A magneticpickup comprising a base, a unit mounted on said base including a pairof pole pieces, a voice coil mounted therebetween and an armature in xedrelation between said pole pieces and extending into said coil, a shaftand bearings therefor mounted on said base, a magnetic flux relay pieceattachedto the end of said shaft and having an upstandingV wingextending between the lower parts of said pole pieces to theneighborhood of the lower end of said armature, a stop piece rigid withsaid base and above said shaft and an upstanding post on said shafthaving a fiat upper end extending into a corresponding notch in saidstop piece and damping and limiting the movement of said wing betweensaid pole pieces, said shaft having a stylus clamped near the forwardpart thereof.

9. A magetic pickup comprising a pole'piece, a voice coil and anarmature, said'parts mounted in fixed relation to each other and avibratory assembly including a non-magnetic shaft, a stylus near one endthereof and a magnetic iiux relay piece at the other end thereof withrespect to said stylus, said relay piece including a wing extendingbetween said pole pieces and contiguous to said armature, thepredominating portion of the mass of said vibratory assembly being saidstylus.

10. A magnetic pickup comprising a base, an assembly i-lxed thereonincluding magnetic pole pieces, a voice coil therebetween and a fixedarmature extending into said coil, a coacting magnetic relay assemblycomprising a light tube of non-magnetic metal, rubber bushings thereon,

,' bearings therefor afiixed to said base, a stylus extendingthereacross and clamped with respect thereto, and a magnetic flux relaypiece comprising a shank frictionally fitted into the rear end of saidtube and having an upstanding wing extending between said pole pieces toa point adjacent the armature.

l1. The combination set forth in claim 10 in which a pin is xedtransversely of said tube and has an upstanding flattened end lodged ina notch in a metallic angle piece afxedyto said base above saidbearings.

12. In a magnetic pickup, the combination of a base having lateralstops, a U-shaped magnet supported thereon, a pair of pole piecesaffixed to said base, a leaf spring reacting at its ends against saidstops and exerting pressure on said U-shaped magnet to maintain its endsin contact with said pole pieces, said leaf spring being of widthgreater than the thickness of said magnet and having a longitudinal slitto determine an upper strap bowed outward above said magnet to retainthe same against the bottom of the base.

i3. In a magnetic pickup, a pole piece and armature assembly comprisinga U-shaped pole piece, a non-magnetic metal bar affixed to one leg ofsaid pole piece, and a fixed armature affixed to the edge of said vbarand extending transversely of the legs of said U and magnetically spacedtherefrom,

14. In a magnetic pickup, va magnetic flux relay assembly comprising alight metal tube, a pair of rubber bushings about the ends thereof, ascrew in one end of said tube for clamping a stylus adapted to extendthrough apertures in said tube and a magnetic member having a shankfrictionally fitted in the other end of said tube and having an integralwing upstanding therefrom.

a set screw in one endof said tube for clamping a stylus adapted toextend through apertures in said tube and a magnetic member having ashank frictionally fitted in the other end of said tube and having anintegral kwing upstanding therefrom, and a positioning pin axedtransversely through said tube and having anupstanding flattened end inthe plane of said wing.

16. In a magnetic pickup, a base, a magnet pole piece, coil and fixedarmature assembly thereon, a vibratory assmbly co-ordinated therewithand comprising a magnetic flux relay piece including' a wing extendingbetween said pole pieces and contiguous to said armature, a nonmagnetictube mounting said wing, bearings for said tube mounted on said base, astylus transversely of said tube, means clamping said stylus between itsends, the ends of said stylus being of diameter and mass greatly reducedwith respect to the median clamped portion thereof.

l?. Amagnetic pickup comprising a case having a removable metal coverthereon, a stylus carrying tube longitudinally of said case, a vibratorymagnetic member mounted on said tube, pole pieces in said casecorrelated therewith, a stylus transversely through said tube, anaperture through saidv cover with which the upper end of said stylus isflush when properly positioned and a set screw through the forward endof saidl case for clamping said stylus with re,- spect to said tube.

18. A magnetic pickup comprising a base having a cover, a magnet, anarmature and a voice coil thereabout mounted on said base, a magneticflux relay piece extending between said pole pieces and contiguous tosaid armature, a hollow shaft mounting said relay piece and havingbearings ahixed to said base, a stylus transversely through the forwardbearing and Athrough said shaft, said stylus having an enlargementcontacting the upper portion of said forward bearing when positionedthrough a corresponding aperture in said cover and a set screw throughthe forward end of said base clamping said stylus in position.

19. The method of assembling a magnetic pickup of the. type comprisingpole pieces, a fixed armature and a flux relay piece extending betweensaid pole pieces and adjacent said armature, said method comprising thecentering of said ux relay piece between the pole pieces by straddlingsaid relay piece with a'fork fitting snugly between said pole pieces,clamping the mount for said flux relay piece in thus centered position,mounting the armature piece in contact with said iiux relay piece, andthen interposing a shim of appropriate thickness between-the pole piece'and the armature mount to elevate said armature with respect to saidflux relay piece to determine the desired air gap between the stationaryarmature and the flux relay piece.

20. 'A magnetic pickup including a magnet having pole pieces and havinga xed armature, a voice coil encircling part of the magnetic circuit, amagnetic flux relay piece associated with said pole pieces and saidarmature, a stylus, a shaft 'adjacent to said relay piece and rigidtherewith rigidly connected to an end of said shaft for impartingvibration to the relay piece to cause the alternations and variations ofmagnetic flux through said coil, said armature being concave at the partcontiguous to said ux relay piece for reductionof the air gaptherebetween in a direct' ratio with the amplitude of vibration of saidflux relay piece.

2l. A magnetic pickup including a magnet having pole pieces and having aiixed armature, a voice coil encircling part of the magnetic circuit, amagnetic flux relay piece asso- 1o ciated with said pole pieces andadjacent one end of said armature, a shaft adjacent to and carrying saidrelay piece, a stylus at anV end of said shaft, said armature having ahollowed out conformation in the region of said flux relay piece tocause the latter to approach one or the other lateral portions of saidarmature and reduce the air gap in a direct ratio with the amplitude ofvibration of said relay piece. Y

MAXIMILIAN WEIL.

